Scientific theories are created to explain evidence.Evidence is used to decide between rival theories: not support a theory.This difference matters.Most scientists don’t know this, just like...Pilots don’t need to know all the details of how the engines work to fly a plane.Example: The observation that Mercury is at position A on the sky and not B is evidence.This evidence can be used to show Newton’s Theory of Gravity is false.The only theory left that explains that piece of evidence along with everything that Newton’s theory does and lots more besides is the General Theory of Relativity.

No one knows the details of how all the systems of an Airbus A 380 work. The pilot doesn’t need to know the intricacies of how to fix a broken engine, or how to reprogram a software glitch in the autopilot or how the optic fibre for the inflight entertainment transmits its signals. Sure, s/he might have a very general idea of how these systems function - but the Captain of the plane is, typically, not the person you want to send to actually fix these things during maintenance time. They lack the expertise. Of course, similarly if the autopilot does shut down mid flight due to a glitch, you don’t want the ground engineer or the in-flight entertainment electrician to be the one landing the plane. They typically don’t have the expertise.

The pilot can take off, fly and land - have the aircraft do exactly the primary job it needs to - without knowing precisely how those massive Rolls-Royce engines work. The details of how the engines work, and what to do to fix them when they don’t require their own experts - but they can be operated by pilots who don’t know all the details. Just as the typical person behind the wheel of a car doesn’t need to know the mechanics or the chemistry of how an internal combustion engine functions.

But things do go wrong. And when they do, this is where knowing a few more details can be the difference between solving your problem and making things worse. If an engine on a big passenger jet does shut down at 40,000 feet because of a fire or collision with a bird, the pilot is going to be reaching for one of the manuals and checklists stored in the cockpit explaining some of the finer details of what to do. Suddenly very specific knowledge of engines becomes important. Do they cut the fuel first, or pull the emergency fire lever? Should they climb higher to give themselves more altitude and time incase of more shutdowns, or should they descend and hope increased oxygen helps? Should they attempt to restart an engine that has shut down or not? These concerns are the 0.001%. Such scenarios are prepared for in great detail - but in the end, the pilot needs to be an expert on flying the plane. Not on mechanics. Although it would be great if they were both.

Similarly if you break down on a dirt road in the Australian desert and you’ve not seen another car for a day - knowing a little bit about mechanics could get you back on the road again - and possibly save your life. Being the best rally-car driver in the world is no use whatever if the engine won’t start.

So it is with science. The professional physicist, chemist, biologist - whoever - can do their job day-to-day without ever bothering to know the finer details of how science actually accomplishes what it does. An astronomer can be given time by NASA to use the Hubble Telescope and make observations no one else ever has before of galaxies billions of light years distant. They can write a paper detailing how they have calculated the distance to a cluster of galaxies more precisely than any previous measurements. And they can do this all as well as anyone has before without ever encountering a problem in getting that paper published. And yet they might not know something as simple as: what is the actual purpose of evidence they gather? What is the purpose of their observation? They are making observations each and every day. They are gathering lots of evidence with their telescope and they are writing papers about it - but unless they have certain other expertise they might never be able to correctly and in detail answer the question: what is evidence for? And most of the time - like the pilot or the driver - they can get away without knowing or with a bad very general idea of what’s happening. Because they don’t need to know.

The academic field that purports to explain how science works is known as the Philosophy of Science and is not a part of most science degrees. And this field has a deservedly bad reputation. Like most philosophy courses what is done is a historical overview of major debates. One concentrates on what particular philosophers had to say on the topic and the field is discussed as if no conclusions were ever reached and no particular idea ever shown to be false. This is wrong. It’s not like science where we rarely consult the original texts. When I learned physics I never actually read what Einstein had to say about special relativity nor did I learn about gravity directly from Newton’s Principia Mathematica. And the reason is that today we know more about Special Relativity than Einstein did and more about gravity than Newton did. Sure - they discovered those things and were geniuses to do so and so are rightly famous. But this does not mean they had the final word. We actually understand more now and we actually understand how to explain things better than they did too.

But in philosophy class you go straight to the original. And it can be hard to read. And it’s a perverse system where you have to struggle through old English, as though it was a literature class rather than a class about ideas. When reading Descartes or Hume I always thought it was great stuff - but at the same time I always thought: can’t someone explain this stuff even better? In fact that often became what your philosophy essays were about: trying to explain those ideas in your own words. And then you yourself made progress on those ideas: improving them through criticism. And you managed to come up with your own, better ideas. I wondered: why didn’t the great philosophers of today improve on Decartes and Hume? Why weren’t they more like the great physicists who made progress? Was no progress being made? It was - I just didn’t know it because you don’t find that out in a university philosophy lecture. You have to go elsewhere.

Huge progress has been made in the philosophy of science. We really do know more about precisely how science creates knowledge now than we have ever done in the past. We have learned, for example, that observations are important, that the role of evidence is to decide between rival theories, that induction is false and that authority should be rejected. Most scientists have a general overview of how this works but they are also apt to hold in their minds major misconceptions about the whole process. But it does not affect them most of the time (just like ignorance of internal combustion engines is not a problem for the driver most of the time) - but just now and again ignorance has a way of revealing itself and preventing progress and causing problems.

University marketing departments are fond of telling potential students that a science degree teaches you how to think critically. But it almost never does. At least not explicitly (modulo some rare exceptions). A chemistry lecture is not a critical thinking seminar. Critical thinking means thinking about ideas - not merely learning new ideas. Learning chemistry (or physics or mathematics) no more teaches you to think critically than a lecture on politics teaches you mathematics. They are different types of knowledge altogether. Of course an ability to think critically does indeed help one to learn things like chemistry more quickly and we all know how to think critically to some extent and people who do have a propensity for thinking critically can migrate into the sciences (but they also migrate into law and engineering and hairdressing as well). A chemistry lecture might be a critical thinking lesson, only if the student sat there learning the chemistry and at the same time thinking of all the way the chemistry lecture could be wrong. In other words being critical (i.e: criticizing) the ideas themselves. This almost never happens. If you’re a chemistry student - you just want to learn the chemistry - and fast - and pass the exam. You don’t want to think about all the way what you are being taught is misconceived. But there are always misconceptions. How can you spot them? That is what it takes to think critically. Sure some people are more skilled at this than others because somehow, sometime they’ve learned how. Some people really do sit in chemistry lectures and think: “Well that’s not correct. I know that’s not what an atom looks like. And if that’s not what an atom looks like, that can’t be how the electrons transition between orbitals. And if that’s not how they transition, this can’t be why light is produced in emission spectra” (and if you want more detail on exactly that - see my article here).

So a science degree doesn’t teach you to think critically. And a scientist isn’t necessarily a critical thinker who thinks about how it is that science achieves the great things it does. An example of what I mean about scientists typically being ignorant of the details of how science works (and why this is important) is the role of evidence in science. Everyone knows that evidence is important in science (and this is great: knowing that evidence is needed is good - but it’s even better to know why! Knowing that evidence is important for science is like step one. Knowing how evidence is used in science is like step two).

So how is evidence used? What’s it’s purpose? It was Karl Popper who swept away most of what passed for the philosophy of science that went before and built an entire epistemology (a theory of knowledge or way of thinking about thinking) that was able to explain exactly how knowledge is created. His ideas explain science - but also knowledge more broadly. Popper explained how objective knowledge was possible and how science made progress. Now for cultural and political reasons that need not concern us here and now for the moment some people came after Popper (in particular Thomas Kuhn) who went about a systematic undermining of Popper’s ideas. They were very largely successful. Bad ideas can spread like wildfire. In particular what Popper’s opponents asserted was that objective knowledge was not possible and that science merely told stories (narratives) where one idea was no better than another but that fashions changed and people merely changed their ideas according to which paradigm they were operating in. This appeals to people who are threatened by science.

But of course those people are wrong. Science does not merely change according to whims and fashions. We really do make objective progress and the shortest way of demonstrating that is merely to say: look around you at how much better things are now than in the past. Scientific theories improve little by little and so we get better medicine, better computers, better aircraft, we can feed more people, we can fix our problems and survive on a hostile planet that is forever trying to kill us with natural disasters.

It is impossible to explain progress if you think that new scientific theories replace old ones simply because it is some group of scientists opinion that this should happen.

Most scientists appreciate this, but they don’t understand it. They’ve heard of Popper - but they don’t fully understand him or his importance. So for example every scientist understands evidence is important - but most don’t know exactly why. In fact most are wrong. And they can get away with it - most of the time.

The common idea is this: evidence is there to support a theory. The theory with the most evidence to back it up is the theory that is the most scientific one and the one we should rely on. More evidence can make a theory more likely to be true, so this idea goes.

But this is wrong. And badly wrong. Sure that idea is a better idea than the idea that: evidence cannot be used at all in science. Instead what we should do is consult some authority (like the most famous scientist in the field, or perhaps some text book, or maybe some sacred scripture). Appeals to authority are even worse than the idea that evidence can be used to support a theory.

So what is the role of evidence? The role of evidence is to decide between rival theories.

If this seems like it’s splitting hairs, it is not. Often there are no rival theories. The evidence has - typically - long since shown all other rivals to the actual scientific theory - to be false.

Let me give you an example from cosmology. One of the most successful theories to explain the motion of the planets across the sky is Newton’s Theory of Gravity. Newton discovered one equation in particular - the universal law of gravitation - that could be used to predict where the planets would be at any time of the night (or day) in the sky. It could be used to predict the position of the moon, and the sun and the tides. It could be used to predict how fast things fell to the ground when dropped.

But eventually it was discovered that there were some things that Newton’s Law of Gravity failed to do. It failed to properly predict the position of Mercury over time. To be precise, it failed to account for the precession of Mercury’s orbit. Orbits are not perfect circles - but they are oval shaped (ellipses). This means there comes a time in a full orbit where the planet is closest to the Sun. This point of closest approach moves over time - and this is called “precession”. Now Newton’s theory allowed the precession to be calculated - but it gets the position wrong.

So that’s some evidence: the position of Mercury in the sky. And here is a theory: Newton’s Law of Gravity. So what is the role of the evidence? The role of the theory was to explain the evidence. The role of the evidence? Is it to support the theory?

Until there is a rival theory, the evidence goes unexplained and is called a problem or maybe a mystery or something like that. But if there is a rival theory then the evidence can be used to decide between the two theories - to decide which is better and which can be said to be false.

Now here is the key: if we never created a new theory but simply made more and more precise measurements of the position of Mercury - then this would be a problem. We might think: our measurements are wrong. There are lots and lots of results like this all the time in science. This is what makes science great: the countless problems. At the moment we have questions in astronomy alone like: what is dark matter? What is dark energy? Are the laws of physics the same here as they are on the other side of the universe? In each of those cases it could always be the case that the measurements we have made contain errors.But it also might be the case that our theories are wrong. So back to Newton: imagine yet another piece of evidence came along. Let’s say Newton’s Theory of Gravity does something like make a prediction that when light from a distant star passes by the Sun during a solar eclipse that it should only be bent by “x” amount. Now say we do the measurement. And say it’s not bent by x amount but by 2x amount. Double what is predicted. What then?

Well the evidence would be mounting against Newton. Still we might think that our measurements are wrong. That we are missing something. But two, quite different, experiments that Newton’s theory cannot account for makes it harder for us to keep thinking that Newton’s theory is the final word on gravity.

Initially, by the way - if you look at the actual history of this - there were no rivals. The evidence gathered about Mercury and the bending of starlight by the Sun constituted a problem. A mystery. So what was the role of the evidence? Well the role of evidence there is to cry out for an explanation. A creative explanation. In other words a new scientific theory.

Eventually one did come. It was Einstein’s General Theory of Relativity. The General Theory of Relativity explained that Newton’s Law of Gravity was an approximation to something else far more - well “General”. The General Theory united ideas about light, and magnetism and electricity (from the special theory) with ideas about space and time (and so gravity). And, here’s the key: it fully accounted for the motion of Mercury and it predicted exactly where light should be when it passed by the Moon during a solar eclipse. It got all those things right.

And what became the role of the evidence from Mercury and starlight then? The evidence that Mercury was here in position A (as predicted by Einstein’s General Theory of Relativity) and not there in position B (as predicted by Newton’s Theory of Gravity) - well the role of evidence there is simple. It’s to decide between those two theories. So at the moment of time in the past - in 1915 when Einstein first published his theory we had, briefly, two theories that purported to explain the nature of gravity. How to decide? Let the evidence decide.

Now it’s not that the evidence “supported” Einstein. No. It simply rejected Newton.If the “support” idea was true - then what happened to all that support that Newton’s theory of gravity gained over the hundreds of years prior? How can we make sense of that? If it was being “supported” each and every day by observations of the planets in the sky, the tides going in and out, apples falling to the ground - then did all that support count for nothing? That all makes no sense because the entire philosophy of “evidence as support for a theory” is false. Newton’s theory explained all of that stuff about planets, tides and apples in a particular way. And one way of checking how good the explanation was, was to check the predictions. And for a long while they checked out. Until they didn’t. And once they didn’t we had a problem. And once we had another theory, we were able to decide which theory was better and show, definitively, which theory was false - and how.

It is possible (indeed it is required) that General Relativity is not the final word on gravity. Indeed we know it cannot be the final word because General Relativity makes some predictions about the nature of reality that conflict with what quantum physics says. In other words, these two great theories disagree. So we know neither are the final word. So no evidence “supports” them. It is just that those two theories are the very best scientific theories we have. There is none better, and so no others we can rely on. If you want to build a GPS system, or explain what’s going on in a galaxy far far away - General Relativity is absolutely indispensable. Whatever the “ultimate truth” happens to be - General Relativity is closer to it than anything else we currently know about.

Just to labour the point: there is no better theory of gravity than Einstein’s General Theory of Relativity. It is an amazing theory. The equations of general relativity are the reason why you managed to find that bar last Friday night. The GPS on your iPhone is accurate to a distance of around 2 meters. If we programmed it with Newton’s Theory, it would be out by 30 m on the first day, 100m on the second day and it’d get worse from there. There are (currently) no rivals to General Relativity. But is it the final word? No. And we know that too. That’s exciting.

We know it’s objectively better - closer to the truth - than anything else that has ever been thought on the subject of gravity so far. General Relativity isn’t part of a “paradigm” that has equal value with other “paradigms”. It is not a “narrative” about reality or a “story” that we tell. It is a factual description of what is really there. Now we know it cannot all be correct. But we also know it cannot all be false. We will eventually create a better theory. But we won’t do away with it altogether - just as we did not altogether do away with Newton.

Now not only is Newton still good enough to do all the things it used to (predict where the Sun and moon and most other planets will be, predict the tides and the fall of apples) but it can even be used perfectly well to get rockets to the Moon. But Newton’s Theory got a lot of stuff correct: gravity really does following (approximately) an inverse square law. It’s not an inverse cube law or something like that. The gravity does depend on the mass of things and it gets stronger as you get closer and results in things like elliptical orbits. So there is so much Newton got correct.

But in the end the evidence not only did not support Newton (because it never does do that) - the evidence falsified Newton. It shows that Newton’s Law of Gravity was, strictly, false. It was a closer approximation to what had gone in the past, but ultimately, though it was a closer shot - something else was closer still to the bullseye.

Why does any of this matter today?

Well that whole story is known by all physicists and astronomers. But almost all physicists and astronomers do not know the lesson. It’s about the role of evidence: to decide between rival theories.

Positive support for a theory simply does not exist. Scientific theories - like ideas more generally - are creative conjectures. They are guesses. Not merely wild guesses - guesses which explain the evidence. And when they fail to explain the evidence, you have a problem. Maybe your observations were wrong. Or maybe your theory is wrong.